In a stepper type or step-and-scan type projection exposure apparatus which is used for transferring an image of a pattern of a reticle used as a mask onto a wafer (or a glass plate) used as a substrate via the projection optical system when a semiconductor device is fabricated, exposure must be performed while constantly maintaining the image formation characteristics of the projection optical system in good status. This is why the projection exposure apparatus has been stored in a chamber where temperature and humidity are controlled and air from which particles are removed by a dust proof filter is circulating, and temperature and humidity around the projection optical system have been set to a predetermined status.
However, the barometric pressure of the air inside the chamber changes in association with the atmospheric pressure, so if the atmospheric pressure of the location where the projection exposure apparatus is operating changes, the barometric pressure of the air around the projection optical system and the air between lenses in the projection optical system changes, and the image formation characteristics of the projection optical system, such as the best focal position and the projection magnification, change slightly. If exposure is performed in the status where the image formation characteristics have changed in this way, the resolution of the image to be projected onto the wafer deteriorates, or an alignment error is generated. Therefore conventionally the relationship of the fluctuation amount of the image formation characteristics of the projection optical system with respect to the barometric pressure around the projection optical system (hereafter “atmospheric pressure dependency”) is measured and stored in advance when the projection optical system is assembled and adjusted for example, and when the projection exposure apparatus is operating, the barometric pressure around the projection exposure apparatus, for example, is constantly measured, the fluctuation amount of the image formation characteristics of the projection optical system is determined based on the measurement result and the stored relationship, and the fluctuation amount of the image formation characteristics is corrected.
In order to measure the atmospheric pressure dependency of the image formation characteristics of the projection optical system, the barometric pressure around the projection optical system has been changed at assembly and adjustment in the following ways; (a) waits for the atmospheric pressure to gradually change over a long period of time, and (b) storing the projection optical system in a pressure chamber where internal barometric pressure can be changed to a desired value.
However, of these methods of changing the atmospheric pressure, method (a) of waiting for the atmospheric pressure to change takes a long period of time for assembly and adjustment, and method (b) of using a pressure chamber increases the cost of manufacturing equipment, so in both cases, it is difficult to apply one method to all or all kinds of projection optical systems. In the case of method (a), a desired atmospheric pressure change characteristic may not be obtained because the change of atmospheric pressure is subject to natural phenomena.
Recently, a huge projection optical system, such as a cata-dioptric system, is used to respond a shortening of the exposure wavelength and an increase of the numerical aperture. If method (b) is used for such a huge projection optical system, a large pressure chamber is required, and manufacturing cost further increases accordingly.
With the foregoing in view, it is a first object of the present invention to provide an adjustment method for a projection optical system which allows substantially to measure the relationship (dependency) of the fluctuation amount of the image formation characteristics of the projection optical system with respect to the change of the installation environment (e.g. barometric pressure) around the projection optical system, without actually changing the installation environment.
It is a second object of the present invention to provide an adjustment method for a projection optical system which allows to substantially adjust the projection optical system according to the installation environment (e.g. atmospheric pressure) of the installation location where the projection optical system is actually used for exposure without changing the installation environment of the assembly location where the projection optical system is assembled and adjusted, when the installation environment is different between the assembly location and the actual installation location (relocated location).
It is a third object of the present invention to provide an adjustment method for a projection optical system which allows to very accurately correct the change of the image formation characteristics of the projection optical system, which is generated due to a fluctuation of the installation environment.
It is a fourth object of the present invention to provide an exposure method using such an adjustment method, and a projection exposure apparatus which can execute the adjustment method. It is still another object of the present invention to provide a device fabrication method using such an exposure method.